1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for use in laser beam printers, facsimiles and digital copiers, and to an image forming apparatus and a process cartridge therefor using the electrophotographic photoreceptor.
2. Discussion of the Background
In place of conventional inorganic materials such as Se, CdS and ZnO as photoconductive materials for electrophotographic photoreceptors, organic photoconductive materials having better sensitivity, thermostability and nontoxicity than the inorganic materials therefor have widely been developed, and many copiers and printers are now equipped with electrophotographic photoreceptors using organic photoconductive materials. When a photosensitive layer of the electrophotographic photoreceptors using organic photoconductive materials is formed, a functionally-separated photosensitive layer including a charge generation layer (CGL) and a charge transport layer (CTL) overlying the CGL is widely used because of having good sensitivity and durability. Typically, image forming apparatuses such as printers, copiers and facsimiles form images by a successive process of charging, irradiating, developing, transferring and fixing, and as electrophotographic image forming apparatuses have higher printing speed and higher durability recently, the photoreceptor is strongly required to have reliability to keep producing high quality images even when repeatedly used. Particularly, when an ultrahigh-speed image forming apparatus producing many images stops working to replace photoreceptors, this results in a large deterioration of productivity. In addition, full-color image forming apparatuses mostly using tandem methods, lining four color developing systems in parallel, need photoreceptors having smaller diameters than the conventional photoreceptors to avoid growing in size, and therefore the photoreceptors are required to have higher durability.
One of the abnormal images caused due to the durability of photoreceptors is background fouling in image forming systems mainly using nega-posi development. Sources of background fouling include dirt, a defect of the electroconductive substrate, an electrical insulation breakdown of the photosensitive layer, a carrier (charge) injection from the substrate, increase of darkness attenuation of the photoreceptor and generation of heat carrier in the photosensitive layer. Dirt and defects of the electroconductive substrate can be avoided by excluding such substrates before forming a photosensitive layer thereon, and in a sense, is human-induced and not a substantial source. Therefore, improvement of the voltage resistance, carrier (charge) injection from the substrate and deterioration of electrostatic fatigue of the photoreceptor is thought to be essential solutions.
In consideration of these points, methods of forming an undercoat layer or an intermediate layer between the photosensitive layer and the electroconductive substrate have been disclosed in the past. For example, Japanese Laid-Open Patent Publication No. 47-6341 discloses an intermediate layer formed of a cellulose resin; Japanese Laid-Open Patent Publication No. 60-66258 discloses an intermediate layer formed of a nylon resin; Japanese Laid-Open Patent Publication No. 52-10138 discloses an intermediate layer formed of a maleic acid resin; and Japanese Laid-Open Patent Publication No. 58-105155 discloses an intermediate layer formed of a polyvinylalcohol resin.
However, these intermediate layers including a resin alone have high electric resistivities, and therefore the residual potential increases, resulting in occasional deterioration of image density and image gradation in negative and positive images. In addition, the intermediate layers have ion conductivities due to impurities and have particularly high electric resistivities, and therefore the residual potential noticeably increases. Therefore, the intermediate layers need to be thinner, resulting in insufficient chargeability of a photoreceptor after repeated use.
In order to control the electric resistivity of an intermediate layer, methods of dispersing an electroconductive additive are disclosed. For example, Japanese Laid-Open Patent Publication No. 51-65942 discloses an intermediate layer wherein a carbon-containing or a chalcogen-containing material is dispersed in a hardening resin; Japanese Laid-Open Patent Publication No. 52-82238 discloses a heat-polymerized intermediate layer formed by using an isocyanate hardener including a quaternary ammonium salt; Japanese Laid-Open Patent Publication No. 55-113045 discloses a resin intermediate layer including a resistivity regulator; and Japanese Laid-Open Patent Publication No. 58-93062 discloses a resin intermediate layer including an organic metallic compound. However, these resin intermediate layers alone occasionally have problems of producing moiré images due to light interference in image forming apparatus using coherent light such as a laser beam.
In order to prevent the moiré images from being produced as well as to control the electric resistivity of an intermediate layer, photoreceptors having an intermediate layer including a filler are disclosed. For example, Japanese Laid-Open Patent Publication No. 58-58556 discloses a resin intermediate layer wherein aluminum oxide or tin oxide is dispersed; Japanese Laid-Open Patent Publication No. 60-111255 discloses an intermediate layer wherein an electroconductive particulate material is dispersed; Japanese Laid-Open Patent Publication No. 59-17557 discloses an intermediate layer wherein magnetite is dispersed; Japanese Laid-Open Patent Publication No. 60-32054 discloses a resin intermediate layer wherein titanium oxide and tin oxide are dispersed; and Japanese Laid-Open Patent Publications Nos. 64-68762, 64-68763, 64-73352, 64-73353, 1-118848 and 1-118849 disclose resin intermediate layers wherein powders of borides such as calcium, magnesium and aluminum; nitrides; fluorides; and oxides are dispersed. The filler-dispersed intermediate layer needs to include a large amount of the filler (namely, to decrease the amount of the resin) because the potential properties of the intermediate layer depends on the amount of the filler. As the resin decreases, the adhesiveness thereof to the electroconductive substrate decreases, resulting in peeling of the intermediate layer from the substrate. Particularly, when the substrate is a flexible belt, this problem is noticeable.
In order to solve this problem, a multilayered intermediate layer is disclosed. The multilayered intermediate layer is broadly classified into two types. One includes a resin layer wherein a filler is dispersed on an electroconductive substrate and a resin layer not including a filler on the resin layer wherein a filler is dispersed (FIG. 1), and the other includes a resin layer not including a filler on an electroconductive substrate and a resin layer wherein a filler is dispersed on the resin layer not including a filler (FIG. 2).
The former multilayered intermediate layer includes an electroconductive resin layer wherein a low-resistivity filler on an electroconductive substrate to hide defects thereof, and the above-mentioned resin layer is formed on the electroconductive resin layer. These are disclosed in, e.g., Japanese Laid-Open Patent Publications Nos. 58-95351, 59-93453, 4-170552, 6-208238, 6-222600, 8-184979, 9-43886, 9-190005 and 9-288367. These have the same electrostatic defects as the photoreceptor having an intermediate layer including a resin alone because a lower electroconductive layer plays a role of an electrode of the electroconductive substrate. Only, the photoreceptors have moiré prevention capability because the electroconductive layer is a filler-dispersed layer scattering writing light. When the photoreceptor is charged, a charge having a reverse polarity to that of the surface thereof reaches an interface between the lower (electroconductive) layer and the upper (resin intermediate) layer. However, when the electroconductive layer does not have so low resistivity, the charge injection from the electrode is not fully performed, and the residual potential largely increases because the lower layer becomes a resistive component. Particularly, the lower layer needs to have sufficient thickness not less than 10 μm to cover defects of the electroconductive substrate, and this problem noticeably occurs.
On the other hand, the latter multilayered intermediate layer includes the resin layer on an electroconductive substrate, and a resin layer wherein a low-resistivity or an electron-conductive filler is dispersed on the resin layer. These are disclosed in, e.g., Japanese Laid-Open Patent Publications Nos. 5-80572, 6-19174 and 2005-128495. These having positive-hole blocking capability is effective in background fouling. In addition, since the upper layer is a filler-dispersed layer, the residual potential is less accumulated. Since charge (positive-hole) injection from the electroconductive substrate into the photosensitive layer can be prevented, background fouling can considerably be reduced in nega-posi development. In addition, the charge blocking layer below can reduce residual potential more than the above.
Resins for use in the intermediate layers need the following capabilities:
(1) solvent resistance such that it is not easily soluble or deformed in a solvent used for coating a photosensitive layer including a CGL and a CTL on the intermediate layer;
(2) uniform coated layer without coating defects to obtain electrical barrier or not to influence the coatability of the upper layer; and
(3) good adhesiveness to the electroconductive substrate.
For these purposes, a polyamide resin, particularly a N-alkoxymethylated polyamide is preferably used in the intermediate layer. For example, Japanese Laid-Open Patent Publication No. 9-265202 discloses a method of including an alkoxy methylated copolymer nylon resin having an alkoxy methylation of from 5 to 30% in an undercoat layer; Japanese Laid-Open Patent Publication No. 2002-107984 discloses a method of including an inorganic pigment and a crosslinked N-alkoxy methylated polyamide resin as a binder resin in an intermediate layer; Japanese Patent No. 3086965 discloses a method of including a N-alkoxy methylated polyamide copolymer mainly constituted of a λ-amino-n-lauric acid in an intermediate layer; and Japanese Patent No. 3226110 discloses a method of including a polyamide resin including a unit having a structure in an intermediate layer. Methods of including a N-alkoxymethylated nylon in an undercoat layer or an intermediate layer are known, and effective in preventing charge injection from the electroconductive substrate to prevent background fouling.
However, although the above-mentioned plural undercoat layers or intermediate layers including N-alkoxymethylated nylon are effective in preventing background fouling, they have an influence on increase of the residual potential more than a little. Particularly, the increase of the residual potential due to repeated use is serious and is a large problem. When a resin having high hygroscopicity is used, the resistivity of the undercoat or the intermediate layer changes and such photoreceptors tend to largely depend on the environment, e.g., they largely increase in residual potential in an environment of low temperature and low humidity, and are charged less in an environment of high temperature and high humidity, resulting in background fouling. In order to solve this problem, Japanese Patent No. 2718044 discloses a method of including a N-alkoxy methylated polyamide resin, wherein each of Na, Ca and P atoms which are impurities has an elemental concentration not greater than 10 ppm in an undercoat layer; Japanese Laid-Open Patent Publication No. 6-93129 discloses a method of including N-methoxymethylated nylon 6 including components having a molecular weight not greater than 1,000 not greater than 10 ppm therein; and Japanese Patent No. 2887209 discloses a method of using an alcohol-soluble nylon contacted to a mixed solvent including alcohols and ketones. These remove impurities included in the N-alkoxymethylated nylon resin to solve electrostatic problems.
However, although the method in Japanese Patent No. 2718044 prevents defective images when repeatedly used in an environment of high temperature and high humidity, the resistivity of the undercoat layer increases and the residual potential inevitably increases in an environment of low temperature and low humidity. The method disclosed in Japanese Laid-Open Patent Publication No. 6-93129 can prepare a photoreceptor having long stability at room temperature, but the chargeability deteriorates and the residual potential increases in an environment of low temperature and low humidity or high temperature and high humidity, resulting in background fouling. The method disclosed in Japanese Patent No. 2887209 can prepare a photoreceptor having stable potential contrast in an environment of low temperature and low humidity, and of high temperature and high humidity, but still has problems in durability for long periods when having a small diameter or installed in a high-speed image forming apparatus.
Japanese Laid-Open Patent Publications Nos. 2005-128495, 2005-128496 and 2005-128497 disclose a photoreceptor that is much improved in background fouling after use for long periods, including at least an intermediate layer and a functionally-separated photosensitive layer on an electroconductive substrate, wherein the intermediate layer includes an antimoire layer and a charge blocking layer, and wherein the charge blocking layer includes an alcohol-soluble nylon AMILAN CM8000 from Toray industries, Inc. and the photosensitive layer includes a CGL including a titanylphthalocyanine crystal having an average primary particle diameter not greater than 0.25 μm as a charge generation material, having at least a maximum diffraction peak at a Bragg (2θ) angle of 27.2±0.2°; main peaks at 9.4°, 9.6° and 24.0°, a minimum diffraction peak at 7.3°; not having peaks at greater than 7.3° and less than 9.4°; and further not having a peak at 26.3° when irradiated with a specific X-ray of CuKα having a wavelength of 1.542 Å. However, the alcohol-soluble nylon AMILAN CM8000 from Toray industries, Inc. has a conductivity less than 1.5 μS/cm and high hygroscopicity, and the resistivity stability and potential contrast stability in an environment of high temperature and high humidity still need improvement.
The N-alkoxymethylated nylons are insoluble in alcohol solvents with some exceptions thereof. When the N-alkoxymethylated nylon is used in an intermediate layer, the N-alkoxymethylated nylon dissolved therein is used. However, a coating liquid including the N-alkoxymethylated nylon dissolved therein becomes clouded when stored for long periods and at low temperature, and the coating liquid is often coated on a substrate defectively.
Japanese Laid-Open Patent Publication No. 9-152731 discloses a method of including a halogenated hydrocarbon in an alcohol solvent when forming a coating liquid including an alcohol-soluble nylon such that the coating liquid has long storage stability. However, the halogenated hydrocarbon is neither preferable nor practical in terms of an environmental problem.
Japanese Laid-Open Patent Publication No. 2000-56496 discloses a method of including benzyl alcohol in a coating liquid including an alcohol-soluble nylon. Although having a long life, the coating liquid needs a long time to become dry to touch, and tends to be coated unevenly. Further, since the benzyl alcohol has a high boiling point, the drying temperature of the coating liquid needs to considerably be high.
Because of these reasons, a need exists for a photoreceptor with less increase of residual potential and not producing abnormal images such as background fouling and black spots even when repeatedly used for long periods, and stably producing quality images even in an environment of high temperature and high humidity or low temperature and low humidity.